Technical note: Why ifeq: FULLPAGENAME File:Sample.jpg... ?
Because the magic word {{PAGECREATOR}} only works after the page has been published. Therefore it couldn't work in the Licence preview window available at the bottom of the Special:Upload page.
This preview uses a fake page name to work, which is "File:Sample.jpg". Now the template checks wether the page name is File:Sample.jpg or not. If the result is true, it will use {{REVISIONID}} instead of {{PAGECREATOR}} ({{REVISIONID}} = The username of the user who made the most recent edit to the page, or the current user when previewing an edit).
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&lqt_method=thread_history){kind=link}
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&action=info){kind=link}